An antiferroelectric liquid crystal panel is known as having a wide angle of view, a capability of a high speed response, and a good multiplex characteristic, and the studies of the antiferroelectric liquid crystal panel have been energetically carried out. Reference can be made to Japanese Unexamined Patent Publication (Kokai) No. 2-173724.
An antiferroelectric liquid crystal panel has a hysteresis characteristic regarding light transmittance versus applied voltage. Accordingly, when a voltage is applied to an antiferroelectric liquid crystal panel, if the product of the applied voltage and the applied pulse width exceeds a threshold value, a ferroelectric state as the first stable state is selected, if the polarity of the applied voltage is changed, a ferroelectric state as the second stable state is selected, and if the product of the applied voltage and the applied pulse width is below the threshold value, an antiferroelectric state as the third stable state is selected. An example of the characteristic regarding light transmissivity versus applied voltage is shown in FIG. 1. An example of an electrode of an antiferroelectric liquid crystal panel having matrix-forming pixels is shown in FIG. 2. Generally in such an antiferroelectric liquid crystal panel, time-divisional driving is adopted in which the scanning voltages are successively cyclically applied to the scanning electrodes Y1 to Y128, predetermined signal voltages are applied in parallel in synchronization with the scanning voltages to the signal electrodes X1 to X160, and the liquid crystal molecules of the selected pixel are switched in correspondence with the display information.
Various methods of time-divisional driving have been proposed. Examples of the proposed methods are shown in FIGS. 3 and 4. To write one picture plane, the writing of two frames is carried out in which the voltage values of the waveforms of the first frame and the second frame are symmetrical regarding the zero voltage value, so that an alternation of the operation is achieved. The ON state is shown in FIG. 3, and the changes of the voltage and the light transmissivity of pixels at the time of setting the OFF state are shown in FIG. 4. The scanning voltage applied to the scanning electrode consists of three phases in which in the first phase resetting to the OFF state, i.e. the antiferroelectric state, is carried out, in the second phase the state in the first phase is maintained, and in the third phase selecting whether or not the setting to the ON state, i.e. the ferroelectric state is carried out. In the case of FIG. 3, the setting to the ON state, i.e. the ferroelectric state, is carried out, since the third phase of the resultant voltage as the difference between the scanning voltage and the signal voltage exceeds the threshold voltage, while in the case of FIG. 4, the OFF state, i.e. the antiferroelectric state, is maintained, since the third phase does not exceed the threshold voltage.
One of the problems in an antiferroelectric liquid crystal panel is that the response speed of switching from the ferroelectric state to the antiferroelectric state is twice as slow as that of switching from the antiferroelectric state to the ferroelectric state. Therefore, in the prior art method of driving, the period for resetting to the antiferroelectric state is made longer than the period for setting to the ferroelectric state or the antiferroelectric state. However, if the number of the scanning electrode is increased, an disadvantage will occur that the time for writing all the pixels is very much extended. An object of the present invention is to solve the disadvantages in the prior art antiferroelectric liquid crystal panel.
Also, from one of the viewpoints, if the same display is carried out for a long time according to the prior art driving method, some pixels enter the ferroelectric liquid crystal state while other pixels never enter the ferroelectric liquid crystal state, and, when these pixels are switched to the antiferroelectric liquid crystal state, the difference between the layer structures of these pixels appears. This is because the pixels have different layer structures. As a result, the difference between the light transmittances occurs so that a disadvantage of a residual image is brought about. An object of the present invention is to provide an antiferroelectric liquid crystal panel, having matrix-forming pixels, in which the residual image phenomena caused by the difference between the layer structures of the pixels is prevented to ensure a satisfactory display.
Also, from another viewpoint, in order to carry out a satisfactory time-divisional driving in the driving of a liquid crystal display, where the scanning side voltage value of the phase for determining the display state is assumed to be V.sub.C and the signal side voltage value is assumed to be V.sub.D, it is necessary concerning the setting of the voltage to satisfy regarding the first scanning period the relationships: EQU .vertline.V.sub.C .vertline.+V.sub.D .vertline..gtoreq.V5
and EQU 0.ltoreq..vertline..vertline.V.sub.C -.vertline.V.sub.D .vertline..vertline..ltoreq.V1
and, in the time-divisional driving, since the driving is carried out under the condition: V.sub.C &gt;V.sub.D, the range of the value of V.sub.D is considerably limited based on the above-indicated relationships in the case where the liquid crystal material having a large difference between V1 and V5 is used, since the driving is carried out under the condition: V.sub.C &gt;V.sub.D (see FIG. 1). Accordingly, in the case where the liquid crystal material having a large difference between V1 and V5, the range of the setting of the voltage is limited within a considerably narrow allowance, and thus the satisfactory display cannot be realized. An object of the present invention is to set the range of the voltage value during the non-selection period broader than the range in the case of the prior art, and to provide a method for driving an antiferroelectric liquid crystal capable of displaying easily and satisfactorily even if the liquid crystal material has a large difference between V1 and V5.
Also, from another viewpoint, there is a premise that a molecule of an antiferroelectric liquid crystal has three stable states, in which when no voltage is applied the state is the third stable state as an antiferroelectric state, and when a voltage higher than the threshold voltage Vth is applied the switching to the first stable state as a ferroelectric state or to the second stable state as a ferroelectric state, is carried out depending on the polarity of the applied voltage. In the prior art method for the driving, to switch from the ferroelectric state to the antiferroelectric state, the supplied voltage is made 0 volt to cause to switch according to the nature, e.g. the viscosity, of the liquid crystal itself without any application of external forces, and accordingly the response speed from the ferroelectric state to the antiferroelectric state is very low. In the prior art method for the driving, the resetting to the antiferroelectric state is necessarily carried out in the first half of the first phase S1 of the selection period consisting of the first (S1), the second (S2), and the third (S3) phases, and then a selection is made whether the bringing to the ferroelectric state or the bringing to the antiferroelectric state is carried out by the select pulse in the third phase S3 (see FIG. 18). In this method, as described above, the response speed of the antiferroelectric liquid crystal from the ferroelectric state to the antiferroelectric state is low, the bringing to the complete antiferroelectric state cannot take place, if the first phase S1 as the period for the resetting is short, to prevent the satisfactory display. Accordingly, the selection period is required to be sufficiently long, the frame frequency is required not to be so high, the period of the writing to the display plane must be extended, and the driving at the video rate becomes difficult. An object of the present invention is to carry out the resetting to the antiferroelectric state, at a high speed, perfectly, and to provide a driving method for an antiferroelectric liquid crystal element capable of high speed driving.
Also, from a further viewpoint, there is a premise that a molecule of an antiferroelectric liquid crystal has three stable states, in which, when no voltage is applied the state is the third stable state as an antiferroelectric state, and when a voltage having the absolute value higher than the threshold voltage Vs is applied the switching to the first stable state as a ferroelectric state or to the second stable state as a ferroelectric state, switching is carried out depending on the polarity of the applied voltage. The switching from the ferroelectric state to the antiferroelectric state is very slow. An object of the present invention is to carry out perfect resetting to the antiferroelectric state in the selection period, and to provide a driving method for an antiferroelectric liquid crystal element capable of a high speed display.